This invention relates generally to combustion of fuels and more particularly to automatic systems for controlling the combustion of fuels. More specifically, the invention relates of an automatic fuel combustion control method and system by which, in an apparatus wherein a fuel which contains therein one or more kinds of combustible components, and in which, while the compositions of the one or more combustible components are known, the mixture ratio of only one of the combustible components is unknown, is undergoing combustion, the fuel is caused to undergo complete combustion as the control system automatically computes the appropriate air flow rate for the combustion.
In an ocean ship for transporting liquefied natural gas (referred to hereinafter by its abbreviation LNG), for example, natural gas which has vaporized is taken out of the LNG tanks and burned in a boiler in order to maintain the internal pressures in the LNG tanks within an allowable range. This natural gas contains methane constituting a combustible component and nitrogen constituting an incombustible component. Since the vaporization temperature of the nitrogen is lower than that of the methane, the quantity of the nitrogen is relatively large and the mixture ratio of the combustible methane is low in the gas thus taken out of the LNG tanks soon after the LNG tanks have been loaded with the LNG. However, with the elapse of time, the mixture ratio of the methane increases. The mixture ratio of the combustible component is generally unknown and is said to vary between 60 to 100 percent with time.
Furthermore, since the quantity of the natural gas taken out of the LNG tanks is determined in accordance with the object of maintaining the pressures in the LNG tanks within an allowable range, fuel oil is ordinarily burned simultaneously with the natural gas in order to maintain constant the steam pressure in the boiler as steam is generated at a flow rate demanded by the plant from the boiler. The feed rate of the natural gas, that of the fuel oil, or both feed rates are automatically controlled in accordance with the load (required steam generation rate) of the boiler.
Automatic control systems of known type have been accompanied by a number of problems as hereinafter described in detail.